1. Field of the Invention
The present invention relates to a dome-shaped spring and a switch using the dome-shaped spring.
2. Description of Related Art
It is generally known to use a push button switch with a tactile (click) action for operating an electronic device such as a mobile phone. A push button switch with a click action is a button that can provide a click feel to a user when a user pushes the button. Such a push button switch with a click action is provided with a dome-shaped spring (dome-shaped contact spring).
It is also known an aspheric dome-shaped spring having a smaller size and better click feel than a spherical dome-shaped spring and a switch using the aspheric dome-shaped spring (see JP-2011-34927A). Such a conventional aspheric dome-shaped spring and a switch using the spring will be explained with reference to FIGS. 15 and 16. FIG. 15 is a sectional structure of a conventional switch 100. FIG. 16 shows a contacting between a circumferential edge 21c of a conventional dome-shaped spring 21 and a stationary contact 4.
The switch 100 has, as shown in FIG. 15, a dome-shaped spring 21, substrate 3, and stationary contacts 4, 5 and 6. The dome-shaped spring 21 is a dome-shaped spring for a contact of which the whole surface is aspheric and made of conducting metal. The dome-shaped spring 21 has a circular shape in a plan view and the center of the circle is designated as a movable contact 21a. FIG. 15 is a cross-sectional view along a plane passing through the movable contact 21a of the dome-shaped spring 21.
The substrate 3 is a substrate on which the dome-shaped spring 21 is disposed. The substrate 3 is provided with stationary contacts 4, 5 and 6. The stationary contacts 4, 5 and 6 are electrical contacts made of conducting metal. The stationary contacts 4 and 5 continuously support the dome-shaped spring 21. The stationary contact 6 is located at a position corresponding to the movable contact 21a of the dome-shaped spring 21.
The dome-shaped spring 21 becomes deformed by applying an acting load F onto the movable contact 21a of the dome-shaped spring 21 from vertically upside by a user. By continuing application of the load F, a click action occurs by buckling of the dome-shaped spring 21, and the movable contact 21a becomes in contact with the stationary contact 6. The stationary contacts 4 and 5 are brought into conduction with the stationary contact 6 via the dome-shaped spring 21 in this way. After releasing of application of the load F, by ceasing the pressing by the user, the dome-shaped spring 21 returns to the initial shape. During the repeating movements of the pressing and releasing of the dome-shaped spring 21, the contacting position of the circumferential edge 21c of the dome-shaped spring 21 and the stationary contact 4 displaces in a radius vector direction (side to side in FIG. 16) repeatedly. In the repeated movement, an angle between the dome-shaped spring 21 and the substrate 3 at the circumferential edge 21c is designated as θ2.
A shape of the dome-shaped spring 21 is expressed by the following equation (1) that defines a shape of a neutral plane in a cross-section of the spring without acting load F. The equation is an aspheric equation of a sixth-order even function;f(ρ)=b1·ρ6+b2·ρ4+b3·ρ2+h  (1)where ρ is a radius vector from a plane center (the center point in a plan view) of the dome-shaped spring, b1, b2 and b3 are coefficients, h is a height of the dome-shaped spring from the placing surface (substrate) at the plane center, and f(ρ) is a height of the dome-shaped spring from the placing surface (substrate) at a position of radius vector ρ.
On the placing surface (surface of the substrate 3) of the dome-shaped spring 21, an outer diameter Da is defined as a length between two contacting positions of the circumferential edge 21c of the dome-shaped spring 21 and the placing surface, where a line connecting the two positions pass through the plane center of the dome-shaped spring 21. There are inflection points Pa1 in the equation (1) between the position corresponding to the outer diameter Da and the plane center of the dome-shaped spring 21. A diameter Da1 on the placing surface (substrate 3) of the dome-shaped spring 21 is defined as a length of a line connecting two inflection points Pa1 that passes through the plane center of the dome-shaped spring 21.
In connection with the outer diameter Da, an angle γ1 is defined as an angle which a neutral plane of the dome-shaped spring 21 at the circumferential edge of the dome-shaped spring 21 forms with a surface (placing surface of the dome-shaped spring 21) of the substrate 3 (stationary contacts 4, 5). In connection with the diameter Da1, an angle β1 is defined as an angle which a neutral plane of the dome-shaped spring 21 at the inflection point Pa1 forms with a surface of the substrate 3 (placing surface of the dome-shaped spring 21). The dome-shaped spring 21 satisfies the following equation (2).β1≦γ1  (2)
The equations (1) and (2) mean conditions that a buckling occurs in the dome-shaped spring 21. Because the dome-shaped spring 21 has a shape that satisfies the equations (1) and (2), it can be downsized compared with a spherical dome-shaped spring and click action caused by the buckling occurs without fail. The equation (1) may be an even function of eighth-order or more.
However, as shown in FIG. 16, a contacting force at the stationary contact 4 is large because the circumferential edge 21c of the dome-shaped spring 21 is supported by the stationary contact 4. Therefore, the circumferential edge 21c and the stationary contact 4 tend to become worn by displacement of the circumferential edge 21c side to side caused by the repeated movement of pressing and releasing of the dome-shaped spring 21. In the case where the dome-shaped spring 21 is made of SUS (Steel Use Stainless) and the stationary contact 4 is made of copper foil, the stationary contact 4 becomes worn out because the stationary contact 4 is softer than the dome-shaped spring 21. The same phenomenon occurs at the stationary contact 5. It may cause a conductive failure between the stationary contacts 4, 5 and the stationary contact 6 when the dome-shaped spring 21 is pressed.